Plasma etching has been an indispensable tool in the semiconductor manufacturing industry to precisely transfer mask patterns to underlying layers of solid material. As down scaling of device feature size continues, the challenges faced in many plasma etching processes become exacerbated, including: (i) higher selectivity; (ii) more anisotropic etch (i.e., vertical etch rate must greatly exceed the lateral etch rate due to high-aspect ratio demands); (iii) tighter critical dimension control; (iv) reduced plasma induced damage; and (v) superior throughput. High density plasmas (HDPs), such as electron cyclotron resonance (ECR) plasmas, helicon wave plasmas, and inductively coupled plasmas (ICPs), have been widely used in etching processes to meet some of the previously listed challenges.
However, such HDPs, for example, ICP reactors using a radio frequency (RF) power, source are operated in a continuous wave (CW) RF mode where plasma is excited with constant average power or voltage in a vacuum chamber, which results in potential plasma induced damage (PID) posing a risk to device performance. PID can occur in one or more of the following forms: (i) surface physical damage from highly energetic ions bombarding the wafer; (ii) photon bombardment from high ultraviolet (UV); (iii) photon bombardment from vacuum ultraviolet (VUV) radiation; (iv) plasma non-uniformity induced charging arising from spatial non-uniformity; and (v) differential charging due to negative charging at the top of the high aspect ratio features and positive charging at the bottom.
PID is a major concern affecting micro-electronic and nano-electronic device fabrication. The effects of damage from ion bombardment, UV radiation exposure, and surface charging become more pronounced as the size of devices continues to be scaled down. Time-modulated or pulsed plasmas have been investigated as a means of minimizing the aforementioned damage. Two main parameters characterize the pulse (e.g., an RF pulse): (i) pulse frequency at which the RF power is turned on and off per second; and (ii) pulse duty cycle (DC) defined as the percentage of time which the RF power is on during a single pulse. Pulsed plasmas provide additional “tunable knobs” through which primary plasma properties can be controlled. For example, in an RF pulsed plasma, the supply of RF power to the source and/or bias is switched on and off at set frequency. Additionally, the duty cycle can be varied in a pulsed plasma. Pulsed plasmas, in general, exhibit lower electron temperature, ion energies, plasma densities, and UV radiation than a conventional CW plasma discharge, all of which contribute to reduced damage.